1. Field of the Invention
The present invention relates to an electronic device, a conductive composition, a metal filling apparatus, and an electronic device manufacturing method.
2. Description of the Related Art
In the field of electronic devices such as integrated circuits, semiconductor devices and chips thereof, there has been adopted a method of two-dimensionally arranging semiconductor chips on a circuit board and connecting them by wiring. In this method, however, as the mounting area increases with the number of semiconductor chips, the wiring length also increases, which makes it difficult to achieve compactification, higher performance, and low power consumption of electronic devices. In the present situation where the microfabrication technology has been pursued to the utmost, achieving the compactification, higher performance, and low power consumption through the microfabrication or miniaturization of semiconductor chips cannot be expected anymore.
Therefore, there has been developed a TSV (through-silicon-via) technology where stacked circuit boards are connected together with through-electrodes.
The TSV technology realizes a three-dimensional structure electronic device such as a three-dimensional system-in-package (3D-SiP). This enables incorporation of many functions into a small occupation area and dramatic shortening of important electrical pathways between devices, which results in high-speed processing.
However, the TSV technology has the following several problems.
(1) Insulation between Through-Electrode and Silicon Substrate
As means for electrically insulating a through-electrode from a silicon substrate, Japanese Unexamined Patent Application Publication No. 2008-251964 discloses a technology in which a ring-shaped isolation groove passing through a silicon substrate is formed around a through-electrode passing through the silicon substrate, silicon films are directly formed on bottom and side faces of the isolation groove, and an insulating film is then formed on the silicon films to fill the gap left within the isolation groove, wherein the silicon film whose surface is in contact with an inner or outer peripheral side face of the isolation groove is thermally oxidized into a thermally-oxidized silicon film.
However, even if the through-electrode is electrically insulated from the silicon substrate, reactance may decrease with increase in stray capacitance, particularly in a GHz scale high-frequency area, according to the electrical insulating structure. Thus, a high-frequency signal leaks from the through-electrode to the silicon substrate, deteriorating signal transmission characteristics. In order to improve the signal transmission characteristics in a GHz scale high-frequency area, accordingly, it is necessary to make a further improvement such as increasing the specific resistance as high as possible while decreasing the relative permittivity at the insulating portion where the through-electrode is electrically insulated from the silicon substrate.
If the disclosure of Japanese Unexamined Patent Application Publication No. 2008-251964 is viewed from this point, since the structure is such that the through-electrode is electrically insulated from the silicon substrate by the thermally-oxidized silicon film, the signal transmission characteristics cannot be improved beyond a level that is achieved by the electrical insulation of the thermally-oxidized silicon film. That is, the improvement of the signal transmission characteristics is limited.
Moreover, since it is necessary to have the step of forming the silicon films directly on the bottom and side faces of the isolation groove, the step of forming the insulating film on the silicon films to fill the gap left within the isolation groove after formation of the silicon films, and also the step of thermally oxidizing the surface of the silicon film, the process inevitably becomes complicated and troublesome. When replacing the conventional two-dimensional arrangement technology with the TSV technology, what is important in terms of industrial mass production is the cost/performance, but the above technology cannot sufficiently meet this requirement.
In the above technology, furthermore, since the film is used to fill up the isolation groove, the groove width of the isolation groove has to be set at an extremely small value, for example, about 2 μm, so that considering the typical thickness of a wafer, the aspect ratio of the isolation groove would be as much as 100 to 200. This makes difficult the silicon film formation process for the isolation groove.
(2) Thermal Deterioration of Through-Electrode during Formation and Joining
When forming the through-electrode in a semiconductor chip or wafer, in which a circuit element has been already formed, using the above molten metal filling process or a plating process (via last), it is necessary to avoid thermal deterioration of the circuit element due to heat for melting.
From the viewpoint of avoiding the deterioration of the circuit element due to heat for melting, it is desirable to use a metal material of a low melting point, which however results in deteriorating heat resistance of an electronic device.
For example, tin and indium, which are taken as an example of a metal material for forming the through-electrode in Japanese Unexamined Patent Application Publication No. 2002-158191, have an advantage that deterioration of the semiconductor circuit element due to heat for melting at the time of formation of the through-electrode can be avoided because of the low melting point, but the low melting point impairs thermal reliability.
In order to realize a three-dimensional structure electronic device using the TSV technology, moreover, two or more wafers or chips formed with the through-electrodes have to be sequentially joined together with the through-electrodes brought into alignment. As a junction material, a metal junction material may be chosen from the viewpoint of improving electrical characteristics and joining ability for the through-electrodes. The circuit board can be joined together by melting and then solidifying the metal junction material.
Also in this case, there is a problem that the previously formed circuit element may be thermally damaged during the melting and joining process of the metal junction material.
The same problem also occurs when forming wiring planar conductor patterns on the surface of the wafer along with or independently of the through-electrodes.
(3) Occurrence of Cracks or the like at Through-Electrode and its Surrounding Area
As a common problem among molten metal filling processes, there has been observed a phenomenon that the through-electrode is cracked, the insulating film disposed between the inner wall surface of the through-hole and the peripheral surface of the through-electrode is partially broken by the through-electrode, or eventually the silicon substrate is cracked around the through-electrode.
This problem is not limited to the formation of the through-electrode. Also when stacking a number of circuit boards in order to realize the above three-dimensional arrangement, the same problem may occur at terminals for connection between the circuit boards.
(4) Poor Connection between Through-Electrode and Conductor Pattern
From a functional perspective, the through-electrode has to be connected, at least at one end, to a conductor pattern formed on the substrate. However, if the surface of the conductor pattern is oxidized, poor connection may occur between the through-electrode and the conductor pattern.
As general means for solving this problem, the reduction effect of a flux may be used for reducing the oxide film of the conductor pattern.
However, putting the flux into a minute space along with the molten metal material generates a flux gas. In electronic devices of this type, the minute space is a minute hole having a hole diameter of, for example, tens of μm or less and also a considerably high aspect ratio. If the flux gas is generated within the thus-shaped minute space, of course, the gas cannot easily escape, creating voids due to the flux gas around the through-electrode, which results in reduction of sectional area of the columnar conductor, increase of electrical resistance, and eventually poor connection to the conductor pattern and increase of junction resistance.
This problem is not limited to the formation of the vertical electrode. Also when stacking a number of circuit boards in order to realize the three-dimensional arrangement, it may result in poor connection at terminals for connection between the circuit boards, increase of electrical resistance, and increase of junction resistance.
(5) Difficulty of Molten Metal Filling into Minute Space
When forming the through-electrode, it is extremely difficult to sufficiently fill a high-aspect ratio minute space with a filling material down to the bottom thereof without causing voids or deformation after hardening.
Wafers to be used for manufacturing a semiconductor device are provided with a large number of minute spaces (holes) for formation of electrodes or the like, and the minute spaces are extremely small, for example, with a hole diameter of tens of μm or less. In addition, the thickness of the wafer is considerably large as compared with the minute space having such a small hole diameter, so that the minute space quite often has an aspect ratio of 5 or more. In order to form the through-electrode, a conductive material has to be reliably filled into such a small, high-aspect ratio minute space down to the bottom thereof, which of course requires an advanced filling technology.
As the electrode formation technology, although the technology of using a conductive past that is a mixture of a conductive metal component and an organic binder has been known, attention is now being given to a metallurgical technology using a molten metal material that has superior electrical conductivity, low loss, and excellent high-frequency characteristics. For example, such a technology is disclosed in Japanese Unexamined Patent Application Publication No. 2002-237468 (hereinbelow referred to as Document 1), Japanese Unexamined Patent Application Publication No. 2006-203170 (hereinbelow referred to as Document 2), and Japanese Unexamined Patent Application Publication No. 2002-368082 (hereinbelow referred to as Document 3).
In Documents 1 and 2, at first, disclosed is a technology of filling a metal into a minute space (through-hole) by means of a metal filling apparatus adopting a molten metal refilling process. The molten metal refilling process refers to a process of reducing the pressure of an atmosphere in which a target (wafer) is placed, then inserting the target into a molten metal while keeping the reduced pressure, then increasing the atmospheric gas pressure around the molten metal so that the molten metal can be filled into the space with the difference in atmospheric gas pressure between before and after the insertion into the metal, and then pulling the target out of the molten metal bath for cooling in the air.
In the metal filling apparatus, two rooms, each of which is provided with a pressure increasing/reducing means, are vertically arranged within a chamber and separated from each other by a switching valve. Then, the wafer being a target is held in a suspended state by a carrier jig, dipped into a molten metal bath placed in the lower room, and then moved to and cooled in the upper room for hardening the molten metal within the minute space.
With the metal filling apparatus, however, when the target is pulled out of the molten metal bath, the molten metal within the minute space may be drawn out by the molten metal in the bath or allowed to drip from or bead within the space under the influence of surface tension of the molten metal and the like.
Accordingly, when the target is pulled out of the molten metal bath and then cooled, the surface of the metal within the minute space may be recessed to a level lower than the surface of the target. This may cause defective electrical continuity to the outside.
In order to avoid this, the molten metal has to be supplied again for filling the recess. In order to fill the recess, moreover, the surface of the supplied metal has to be set higher than the surface of the target, which requires a process of matching the surface of the metal with the surface of the target, for example, a CMP (chemical mechanical polishing) process. This may result in complicating the process and causing an ensuing decrease in yield.
A further serious problem is that although the complicated process is required as described above, voids due to insufficient filling of the molten metal may be created in the minute space, particularly at the bottom thereof.
Furthermore, because of its complicated structure, this apparatus is difficult to maintain and unfavorable in view of cost.
On the other hand, Document 3 discloses a metal filling apparatus adopting a differential pressure filling process. In the differential pressure filling process, after a target (sample) formed with a minute space and a metal sheet attached to the target are placed in a vacuum chamber, the pressure of the vacuum chamber is reduced, the metal sheet is melted by heating means, and then the pressure of the vacuum chamber is increased to a level higher than the atmospheric pressure by an inactive gas. With this, the molten metal can be vacuum sucked into the minute space. Subsequently, the vacuum chamber is opened, the molten metal left on the sample surface is removed, and then it is cooled in the air at room temperature.
Document 3 claims that it has an advantage that the sample will never be warped or cracked because its molten metal has a lower heat capacity as compared with the molten metal refilling process (Document 1) and that cost reduction can be achieved because the excess metal can be minimized.
With the differential pressure filling process described in Document 3, however, the molten metal cannot be completely filled into the minute space down to the bottom thereof, which creates voids within.
In addition, since the molten metal left on the sample surface has to be removed, a part (upper end) of the molten metal filled into the minute space may also be scraped off during the process. Accordingly, the problem of the recessed surface remains unsolved.
Moreover, this apparatus is also unfavorable in view of cost and processing efficiency because it takes long time to prepare a metal sheet previously shaped in conformity with the shape of the target and to attach the metal sheet onto the target through a solder ball or the like.
In fact, any wafers manufactured by the differential pressure filling process and devices using the same have not yet been supplied to the market, which proves that the above problem remains unsolved.